日期计算的奇怪错误

Question

I have to calculate how many years passed between two dates. My code works fine - except for very specific dates.

The code bellow should produce the same year difference for both dateEnd and dateEnd1 , however they are not equal. If I would change year from 2003 to 2004, it would become equal. If I would leave the year, but change from march to february, it would again be equal.

I guess it has something to do with leap years, but I am clueless why? Especially since there were 4 leap years in that period, but the year miscalculation happens for dates which are one day apart only.

I expect year difference to be 12 in both of these cases, however I get 13 in the first case with the dateEnd variable

var dateStart = '2003,03,11';
var dateEnd = '2016,03,10';
var dateEnd1 = '2016,03,09';

var difference = (new Date(dateEnd)).getTime() - (new Date(dateStart)).getTime();
difference = (new Date(difference)).getFullYear() - 1970;
alert('Between ' + dateStart + ' and ' + dateEnd + ' elapsed ' + difference + ' years.');
difference = (new Date(dateEnd1)).getTime() - (new Date(dateStart)).getTime();
difference = (new Date(difference)).getFullYear() - 1970;
alert('Between ' + dateStart + ' and ' + dateEnd1 + ' elapsed ' + difference + ' years.');

Here is the code on jsfiddle . One person in the comments managed to reproduce the problem on his jsfiddle !

Here are values of variables I get

dateStart time: 1047337200000
dateEnd time: 1457564400000
dateEnd1 time: 1457478000000
Time difference for the first case: 410227200000
Between 2003,03,11 and 2016,03,10 elapsed 13 years.
Time difference for the second case: 410140800000
Between 2003,03,11 and 2016,03,09 elapsed 12 years.

Answer 1

This is not related to the locale. I also get the 12 / 13 result when I change the dates to any other month in 2016, except January or February. So the reason is that 2016 is a leap year and 2003 is not.

This proves that it is not reliable to calculate date differences as dates. In order to calculate the difference in years, you need to use the year parts of both dates. Also compare the month parts, if the end month is before the start month then subtract one year and if start month and end month are the same then compare the day of the month too. You could also include hours, minutes, seconds and millis, if required.

Answer 2

The can/cannot reproduce difference is not the locale, but the time zone. For me (in California), the above values give:

alert(new Date(410140800000)); // "Thu Dec 30 1982 16:00:00 GMT-0800 (Pacific Standard Time)"
alert(new Date(410227200000)); // "Fri Dec 31 1982 16:00:00 GMT-0800 (Pacific Standard Time)"

Were I at or east of GMT, the second date would be "Sat Jan 1 1983...", giving the different number of years. Leap years are probably coming into it because the number of leap years is affecting the number of days in the difference.

I thought you would see the same thing if you use 2005/2018, but as Arjan points out, it's not quite that simple and does indeed involve leap years.